Fabrication of semiconductor products involves the formation of microelectronic devices on various layers of a semiconductor substrate. To verify the integrity of the fabrication process, a variety of tests and inspections are performed, including optical tests to detect the presence of defects, such as undesired particles and undesired remnants of material layers that were removed during processing. As the size of the semiconductor devices is reduced, the size of the device features to be measured approaches the limits of measurement tools.
U.S. Pat. No. 6,407,373 (the '373 patent) is expressly incorporated by reference herein. The '373 patent describes a two-part inspection method. An in-line inspection and review is performed to detect defects on the semiconductor wafer during the manufacturing process. Classification of defects on the semiconductor wafer involves, the ability to extract accurate information such as defect size, shape, and boundary in order to identify the sources of the defects. An optical measurement is performed in situ, in the processing tool.
The size of the defects to be detected may be below the resolution of the optical instrument, so optical measurements are not sufficiently accurate for pinpointing the locations of the defect and classifying the defect. On the other hand, a scanning electron microscope (SEM) can resolve features with a size of only a few nanometers, but is too slow to use for inspecting the entire wafer. Thus, a two step approach involves initial inline detection of the approximate location of the defects by in situ optical measurement in the processing tool, and incorporation of the approximate locations of the detected defects into a defect map. The processing tool transmits the defect map containing the locations of the detected defects to the SEM. The SEM uses the defect map to inspect small regions containing the locations identified in the defect map, and “re-detects” the defects. During the “re-detection,” the precise location and classification of the defects are determined. By using the defect map, the SEM re-detection takes much less time than would be required for SEM classification without the initial in situ detection. Nevertheless, the duration of the re-detection in the SEM is increased by the presence of systematic errors (rotational and transnational displacements) from the in situ inspection system included in the defect map.
As described in the '373 patent, to eliminate systematic errors of the inspection system from the defect map, a defect offset process is performed by the SEM. The SEM uses the defect map to re-detect a prescribed number of defects (e.g., five defects). The SEM calculates the defect offset, based on the differences between the in situ defect locations in the defect map and the actual locations measured by the SEM. The calculated defect offset is then applied to the remaining points in the defect map to improve the accuracy of the approximate defect locations in the defect map, so that the SEM can re-detect the remaining defects more quickly.
Nevertheless, where the systematic error is relatively large, the amount of time for re-detecting the prescribed number of defects used for defect offset is also large. Further, some types of defects cannot be effectively re-detected by SEM, such as defects buried under (or within) an optically transparent layer.